1. Field of the Invention
The invention is related to display systems. In particular, the present invention relates to driving circuits for liquid crystal display (LCD) systems.
2. Description of the Prior Art
In recent years, LCDs are widely used in personal and commercial products. How to reduce the power consumption of an LCD and its driving circuits, so as to achieve the goal of reducing carbon emission or prolong the usable time of a portable device, has been an important issue for product designers.
As known by those skilled in the art, by providing different voltages to liquid crystal molecules, the rotational direction of liquid crystal molecules can be adjusted. The gray level of each pixel in an image to be displayed is correspondingly controlled. However, the rotational direction of a liquid crystal molecule cannot be fixed for a long time; otherwise the characteristic of the molecule will be destroyed and can no longer rotate corresponding to the voltage. Inevitably, in some practical situations, the image displayed on an LCD must be the same for a long time. To prevent liquid crystal molecules from being destroyed, the driving circuit of an LCD has to continuously adjust the voltages of the display electrodes and the common electrode disposed besides the liquid crystal molecules.
Generally, all the liquid crystal molecules in an LCD share the same common electrode, and the molecules in the same vertical line share one display electrode. When the voltage of a display electrode for a certain molecule is higher than the voltage of the common electrode, the molecule is called as having positive polarity. On the contrary, when the voltage of a display electrode for a certain molecule is lower than the voltage of the common electrode, the molecule is called as having negative polarity.
As lone as the voltage difference between the two electrodes is kept the same, no matter whether the display electrode or the common electrode has the higher voltage, the molecule is corresponding to the same gray level though the rotational directions under these two conditions are opposite to each other. Hence, the driving circuit can change the polarity of liquid crystal molecules between positive and negative alternatively, so as to keep the image the same and the liquid crystal molecules not being destroyed.
There are several ways to alternatively change the aforementioned polarity, for example, continuously changing the voltage of the common electrode. One commonality of these solutions is that the polarity of liquid crystal molecules is changed whenever the image data is changed. For an LCD having an image updating frequency equal to 60 Hz, the driving circuit of the LCD changes the polarity of all the liquid crystal molecules every 16 ms.
FIG. 1 shows an exemplary relationship between an LCD and its driving circuit. In this example, an image driving unit 16 in the driving circuit 10 provides driving signals corresponding to different gray levels to the display electrode 32. The AC voltage generating unit 12 and DC voltage generating unit 14 generates a periodical square wave for the common electrode 34.
As shown in FIG. 1, the AC voltage generating unit 12 is coupled to the common electrode 34 via a coupling capacitor CAC. The coupling capacitor CAC is designed as much larger than the effective loading formed by the common electrode 34. Hence, even if the voltage of the output terminal A of the AC voltage generating unit 12 changes, the voltage difference across the coupling capacitor CAC roughly keeps unchanged. In other words, voltage variations occurring at terminal A will also make the voltage of terminal B, which is connected to the common electrode 34, change. For instance, assume the voltages of terminal A and terminal B are initially 4V and 1V, respectively. If the AC voltage generating unit 12 pulls the voltage of terminal A down to 0V, the voltage of terminal B will then become −3V.
In this example, the output voltage generated by the DC voltage generating unit 14 is kept as VDC; the AC voltage generating unit 12 generates a periodical square wave changing alternatively between 0V and voltage VCAC. Correspondingly, as shown in FIG. 2, the voltage of terminal B (i.e. the voltage provided from the driving circuit to the common electrode 34) will be a periodical square wave changing alternatively between voltages (VDC□0.5*VCAC) and (VDC□0.5*VCAC).
Practically, VCAC is typically twice the supply voltage of the DC voltage generating unit 14 and the image driving unit 16. Therefore, to periodically change the voltage at terminal A and the voltage of the common electrode 34 consumes much power.